package com.example.g5project.tools;

/**
 * @author Hu Zirui
 * @version 1.0.0
 * @ClassName IdService.java
 * @Description TODO
 * @createTime 2022年03月25日 17:05:00
 */
public class IdService {

    /**
     * 起始的时间戳
     */
    private final static long START_STMP = 1480166465631L;

    /**
     * 每一部分占用的位数
     */
    private final static long SEQUENCE_BIT = 12; //序列号占用的位数
    private final static long MACHINE_BIT = 5;   //机器标识占用的位数
    private final static long DATACENTER_BIT = 5;//数据中心占用的位数

    /**
     * 每一部分的最大值
     */
    private final static long MAX_DATACENTER_NUM = ~(-1L << DATACENTER_BIT);
    private final static long MAX_MACHINE_NUM = ~(-1L << MACHINE_BIT);
    private final static long MAX_SEQUENCE = ~(-1L << SEQUENCE_BIT);

    /**
     * 每一部分向左的位移
     */
    private final static long MACHINE_LEFT = SEQUENCE_BIT;
    private final static long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
    private final static long TIMESTMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;

    private long datacenterId;    //数据中心
    private long machineId;       //机器标识
    private long sequence = 0L;         //序列号
    private long lastStmp = -1L;        //上一次时间戳
    private static IdService instance;      // 懒汉式单例

    public IdService() {
    }

    public IdService(long datacenterId, long machineId) {
        if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
            throw new IllegalArgumentException(
                    "datacenterId can't be greater than MAX_DATACENTER_NUM or less than 0");
        }
        if (machineId > MAX_MACHINE_NUM || machineId < 0) {
            throw new IllegalArgumentException(
                    "machineId can't be greater than MAX_MACHINE_NUM or less than 0");
        }
        this.datacenterId = datacenterId;
        this.machineId = machineId;
    }

    /**
     * 产生下一个ID
     */
    public synchronized long getNextId() {
        long currStmp = getNewstmp();
        if (currStmp < lastStmp)
            throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
        if (currStmp == lastStmp) {
            //相同毫秒内，序列号自增
            sequence = (sequence + 1) & MAX_SEQUENCE;
            //同一毫秒的序列数已经达到最大
            if (sequence == 0L) {
                currStmp = getNextMill();
            }
        } else
            sequence = 0L;              //不同毫秒内，序列号置为0

        lastStmp = currStmp;

        return (currStmp - START_STMP) << TIMESTMP_LEFT //时间戳部分
                | datacenterId << DATACENTER_LEFT       //数据中心部分
                | machineId << MACHINE_LEFT             //机器标识部分
                | sequence;                             //序列号部分
    }

    private long getNextMill() {
        long mill = getNewstmp();
        while (mill <= lastStmp)
            mill = getNewstmp();
        return mill;
    }

    private long getNewstmp() {
        return System.currentTimeMillis();
    }

    public static synchronized IdService getInstance() {
        if (instance == null) {
            synchronized (IdService.class) {
                if (instance == null) {
                    instance = new IdService();
                }
            }
        }
        return instance;
    }

    public static void main(String[] args) {
        IdService snowFlake = new IdService(2, 3);
        for (int i = 0; i < (1 << 12); i++)
            System.out.println(snowFlake.getNextId());
    }
}

